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Colour Dodge, Colour Burn, Screen, Overlay, Difference, etc.

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Does anybody know where to find a quantitative description of how these effects are implemented in Photoshop, PSP et al. I have found a qualitative description at http://adscape.com/eyedesign/photoshop/four/xref/glue.normal.html but the actual methods would be nice, before I have to start approximating these descriptions. Thanks for reading. r. "The mere thought hadn''t even begun to speculate about the slightest possibility of traversing the eternal wasteland that is my mind..."

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Guest Anonymous Poster
I once reverse engineered most of those effects but I can only recall two of them:

Difference should be rather easy: Result=Color1-Color2. I don''t know if it''s clamped or not.

Screen (I think, might also be overlay ?) was: Result=Color1+Color2*(1-Color1). Color ranges are from 0 to 1.

If you find some results, please post them here, I wonder if you could implement them in realtime on 3D cards, perhaps through texture shaders.

- AH

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But what are the ''colour'' values ? I take it you have to do it three times, once for each channel ? I think could even do a fast version in software too, using the same techniques as one does to alpha blend quickly etc.

r.

"The mere thought hadn''t even begun to speculate about the slightest possibility of traversing the eternal wasteland that is my mind..."

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Guest Anonymous Poster
Yes, the math was for mono channels, you have to do it for the red, green and blue channel separately. You can do it in software realtime , that''s easy. I was just wondering, if it''s possible to do it on a 3D GPU, so that you could use those effects in realtime 3D scenes. I think it should be possible, I just haven''t found to time to figure it out yet.

- AH

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